Energy from Waste
Waste can really almost be considered to be a cultural asset: remnants of human life have been indicators of social development since human existence. Landfilling was the first waste management concept developed in order to be able to cope with the increasing amounts of waste accumulated in the course of the centuries. This method of depositing waste persisted as the chief element of waste disposal until the end of the 20th century.
The awareness that waste dumping is a ticking time bomb however only really became acute in the 1980s. Concrete political action towards minimisation of the potential damage to the environment by landfill sites only followed in 1993. Dumping untreated waste was then completely forbidden (with a 12 year transition period) by the Technical Instructions on Municipal Solid Waste (TASi).
Two Treatment Methods
The introduction of the Recycling and Waste Management Act (KrW-/AbfG) in 1996 set three objectives: to try to avoid waste, to recycle and dispose of existing waste. These requirements then had to be implemented by all the federal states of Germany.
Exceptions to the rule were to be prevented by the Waste Dumping Directive (AbfAblV) issued 1 March 2001. The obligatory treatment of residual waste made the utilisation of the energetic potential of this waste an important aspect for the legislators. This was however associated with high investments by the operators in safe plants compliant with this requirement. Since 2005, two different TASi-compliant waste pretreatment methods are used in Germany: municipal solid waste incineration (MSWI) and mechanical biological treatment (MBT).
WTE and MBT
Thermal waste treatment with energy recovery and volume reduction
In a thermal waste treatment plant, waste is treated and the energy released during combustion is recovered and used. The volume of the treated waste is significantly reduced in the process.
In the bunker, waste with different characteristics is mixed to create conditions that are as consistent as possible for the subsequent combustion process.
Large grab cranes transfer the waste mixture to the feed hopper, from where it moves onto the combustion grate.
After combustion, the bottom ash is cooled and processed for further treatment. Metals can be separated in downstream processes and sent for further recovery.
The energy released during thermal treatment can be used to generate electricity, district heating and process steam.
Mechanical-biological waste treatment
In mechanical-biological treatment (MBT), waste is first mechanically separated and selected organic fractions are subsequently treated biologically. This process can also reduce the volume of the treated material.
At the beginning of the treatment process, bulky coarse fractions are separated. The material is then processed using shredding and screening equipment to separate different fractions. Metals and other suitable recyclable materials can be separated and sent for further recovery.
Biological treatment of the organic fraction
The organic fraction is prepared for biological treatment, for example by shredding, homogenisation and, where required, moistening. This helps create more consistent conditions for biological degradation.
Different treatment processes can be used. In aerobic treatment, oxygen is supplied. Anaerobic treatment takes place without oxygen and can produce biogas, which may be used for energy generation.
The processes differ, among other things, in terms of aeration, process control and the technical and structural effort required. Material remaining after treatment is further treated or disposed of in accordance with the applicable requirements.
Two treatment methods compared
Under applicable legislation, waste incineration with energy recovery and mechanical-biological treatment are considered equivalent waste pre-treatment routes.
Comparison of Municipal Solid Waste Incineration and Mechanical Biological Treatment.
| Waste Incineration with energy recovery | Mechanical-biological Treatment |
|---|---|
| Thermal treatment of waste | Mechanical separation and biological treatment of selected fractions |
| Energy released during thermal treatment can be used to generate electricity, heat and process steam | In anaerobic biological treatment, the biogas produced can be used for energy generation |
| Metals can be separated from bottom ash in downstream processes | Metals can be separated during mechanical treatment |
| Bottom ash and other combustion residues are further processed or disposed of after treatment | Materials remaining after biological treatment are further treated or disposed of in accordance with the applicable requirements |
| Organic components are treated in the thermal process | Organic components are degradedd in the biological treatment stage |
*TOC = Total Organic Carbon